How do I calculate how much of a object is covered in a 2D image?
In the "Render Layer" / "Passes" panel, enable the Object Index pass. You can then give each object, like your human object, a unique index using the "Pass Index" property in their "Relations" panel.

Once you've given each object of interest a unique "Pass Index", you can inspect that render pass after the image has rendered. Each pixel will have an RGB value equal to the "Pass Index". You can render twice, once with the obstructing objects and once without. Once you've done that, you can find your answer simply by counting counting pixels in each case; dividing the obstructed count by the unobstructed count will give you the unobstructed percentage.